Haskell functions are plotted via a set of tuples obtained form the function.
If you want to make use of gnuplots mighty function plotting functions you can
pass a Gnuplot2D or Gnuplot3D object to plot.

plot X11 $ Gnuplot2D [Color Blue] [] "2**cos(x)"

For 3D-Plots there is a shortcut available by directly passing a String: